Ion implantation techniques for modifying the electrical conductivity properties of semiconductor materials are well known. In order to generate the ions necessary for implantation into the semiconductor wafer, an ion source is provided which generates ions of a chosen element. An extraction assembly comprising a plurality of electrodes is provided downstream of the ion source to extract, accelerate and focus the ions before they enter a mass analyzer and selector and reach the wafer.
U.S. Pat. No. 6,777,882 B2 describes such an ion beam generator apparatus. The contents of U.S. Pat. No. 6,777,882 B2 are hereby incorporated by reference as if set forth in their entirety. FIG. 1 shows an exemplary extraction assembly 11 for generating ion beam 30 and directing the same to a semiconductor wafer disposed in a grounded end station (not shown). Each of extraction, suppression and ground electrodes 23, 24 and 25 respectively, include apertures through which the ion beam extends in this exemplary arrangement. Each of the apertured electrodes 23, 24 and 25 comprise a single electrically-conductive plate having an aperture through the plate to allow ion beam 30 emerging from ion source 20 to pass through. The ion source head includes ion source 20 and arc chamber 20A and is maintained by a voltage supply at a positive voltage relative to ground for a positive ion beam. The energy of the ion beam emerging from the extraction assembly is determined by the voltage supplied to the ion source. A typical value for this voltage is 20 KV, providing an extracted beam energy of 20 keV. Other extracted beam energies, such as beam energies of 80 keV and higher, may be used in other arrangements. Suppression electrode 24 is biased by a supplied voltage with a negative potential relative to ground. For a beam of positive ions, extraction electrode 23 is maintained by a voltage supply at a potential below the potential of the ion source, i.e. source electrode 22, so as to extract the positive ions from the ion source. The potential of extraction electrode 23 would typically be below the potential of the suppression electrode 24 for a low energy beam, and above the potential of suppression electrode 24 for a high energy beam.
The biased extraction electrode 23 is separated from ion source 20 by insulator 44 which contacts both the biased extraction electrode 23 and ion source 20 which is held at the same potential as source electrode 22. In other arrangements, insulators such as insulator 44 can be used to insulate various other high voltage components from ground or other low voltage components.
Such insulators, including insulator 44, can become coated by the gas containing the ion beam and which emanates from ion source 20 and can migrate throughout the extraction assembly and source head. When the surface of insulator 44 becomes coated, the resistance of the insulator decreases and a leakage path is produced between the components it is intended to insulate, in the illustrated example, extraction electrode 23, and ion source 20. This causes the beam current of ion beam 30 to become unstable and eventually requires replacement of extraction assembly 11 in order to maintain performance of the overall ion implantation system.
It would therefore be desirable to provide an insulator that is resistant to becoming coated and providing a leakage path such as that necessitates replacement of the source head components.